1. Field of the Invention
The present invention relates to a flexible circuit board with a simplified structure, and also relates to a method of producing a flexible circuit board having a through hole portion therein.
2. Description of the Prior Art
FIG. 5 shows a conventional flexible circuit board A, which has a five-layered structure including an electrically insulating film substrate 1, a pair of adhesive layers 2a and 2b attached to the upper and lower sides of the insulating film substrate 1, respectively, and a pair of conductive films 3a and 3b respectively fixed to the upper adhesive layer 2a and the lower adhesive layer 2b.
FIG. 6 shows the process of producing the flexible circuit board A. The process comprises two lamination steps. In the first lamination step, the conductive film 3a, on one side of which the adhesive layer 2a has been previously formed, is continuously laid on the upper side of the film substrate 1, and then the conductive film 3a and the substrate 1 are stuck together by the application of heat and pressure using a pair of heat-pressure rollers 4a and 4b. In the second lamination step, the conductive film 3b, on one side of which the adhesive layer 2b has been previously formed, is continuously placed on the lower side of the film substrate 1, and then they are stuck together by means of a pair of heat-pressure rollers 5a and 5b. The adhesive layers may be applied to the film substrate 1 instead of the conductive films 3a and 3b. After the conductive films 3a and 3b and the film substrate 1 are fixed together by the application of heat and pressure, the thus laminated layers are cooled and the adhesive layers therein are dried, resulting in a flexible circuit board A.
The flexible circuit board A thus obtained is then subjected to an etching process so that desired circuits are formed on both sides thereof. Thereafter, a through hole portion is formed therein, resulting in a complete flexible circuit board. The "through hole portion" is an electrical conductive path which electrically connects the upper and lower conductive films of a flexible circuit board.
FIGS. 7 and 8 show conventional through hole portions formed in the flexible circuit board A. Referring to FIG. 7, a hole 7 is first formed in the flexible circuit board A, and the inside wall and the upper and lower rims of the hole 7 are entirely coated with a metal layer 8, resulting in a through hole portion 6a. Referring to FIG. 8, the hole 7 previously formed in the flexible circuit board A is filled with a conductive ink 9, resulting in a through hole portion 6b. Since the metal layer 8 and the conductive ink 9 are conductive, the through hole portions 6a and 6b electrically connects the conductive films 3a and 6b electrically connects the conductive films 3a and 3b.
In a flexible circuit board of the above-mentioned type, since the adhesive layers 2a and 2b are placed between the conductive film 3a and the film substrate 1 and between the conductive film 3b and the film substrate 1, respectively, the adhesive used for the adhesive layers 2a and 2b must have excellent adhesion to both the film substrate 1 and the conductive films 3a and 3b. Thus, the kinds of adhesives which can be used for the adhesive layers 2a and 2b are limited.
Furthermore, the thermal expansion or heat shrinkage of the film substrate 1 is different from that of the conductive films 3a and 3b. Thus, the flexible circuit board tends to be deformed while being cooled and dried after the film substrate 1 and the conductive films 3a and 3b have been bonded together by the application of heat and pressure.
The process of producing such a conventional flexible circuit board requires two separate lamination steps as described above, which complicates the production process.
Furthermore, for the formation of a through hole portion, the step of providing the plating layer or the conductive ink is additionally required after the hole is formed in the flexible circuit board. The increase in the number of steps causes a high production cost.